The present invention relates to a brushless motor that is mainly used in an air blower in an air conditioning system for vehicles.
Brushless motors in the prior, art include the one disclosed in Japanese Unexamined Utility Model Publication No. H2-139473. This brushless motor is provided with a rotor having a field magnet, a stator that generates a rotating magnetic field for the rotor, and a means for exciting the stator. Thus, in this brushless motor, a rotating magnetic field is generated by sequentially exciting a plurality of curved end portions arranged on the stator in a radial direction. The rotor is caused to rotate by the magnet provided on the rotor being repeatedly attracted and repulsed relative to the rotating magnetic field.
In this brushless motor, the means for exciting includes exciting coils that are wound around the stator core, and a means for switching the direction of the current running through the exciting coils. A plurality of field effect transistors (FETS) are normally used as the means for switching. These FETs control the direction of the current running to the exciting coils by controlling signals applied to the gate terminals of the individual FETs. Since a relatively large current runs through the exciting coils, a great quantity of heat is generated, normally reaching up to approximately 150.degree. C., which necessitates that a heat radiator to be provided at each FET.
However, if a problem occurs in the brushless motor itself, e.g. an abnormality in the rotation rate, a circuit error or the like caused by excessive load, it is necessary to protect other normally operating portions from the problem. In particular, it is necessary to cut off communication between the source supply line and the exciting coils to stop the supply of power to the exciting coils so that the rotation of the brushless motor stops.